The Potential Benefits of Greenroofs in Waller Creek Watershed Katherine Jashinski GIS in Water Resources December 1, 2009

Benefits of Greenroofs Reduce sewage system loads by assimilating large amounts of rainwater Reduce sewage system loads by assimilating large amounts of rainwater Absorb air pollution, airborne particulates, and store carbon Absorb air pollution, airborne particulates, and store carbon Insulate a building from extreme temperatures Insulate a building from extreme temperatures Reduce urban heat island effects Reduce urban heat island effects Protect underlying roof material by eliminating exposure to the sun’s ultraviolet (UV) radiation and extreme daily temperature fluctuations Protect underlying roof material by eliminating exposure to the sun’s ultraviolet (UV) radiation and extreme daily temperature fluctuations

Waller Creek Watershed One of the most densely developed streams in the city, >50% impervious cover One of the most densely developed streams in the city, >50% impervious cover 5.63 sq. miles in area 5.63 sq. miles in area Drains to Colorado River at Ladybird Lake Drains to Colorado River at Ladybird Lake

Study Assumptions Roofs are at least one story above grade All roofs are flat 100% of the total roof area is available for conversion to greenroofs Extensive greenroofs

~0.34 sq. miles of the watershed or 6% is viable roof space for conversion to greenroofs

Modeling Runoff Modeling Runoff Simple Method: R = P * Pj * Rv Where: R = Annual runoff (inches) P = Annual rainfall (42.87 inches in 2001) Pj = Fraction of annual rainfall events that produce runoff (usually 0.9) Rv = Runoff coefficient Rv= Ia Ia = Impervious fraction (36.63%)

Benefits of Reduced Stormwater Reduced stormwater load of million gallons for the year Three types of the stormwater benefits are estimated stormwater best management practice savings due to the application of green roofs (BMP) stormwater best management practice savings due to the application of green roofs (BMP) pollutant reduction (P) pollutant reduction (P) reduction of receiving stream erosion (E) reduction of receiving stream erosion (E) BMP$8/sq ft of greenroof $75,338,588 Pollutant ReductionP=05.*$1,414,133/sq mile green roof$238,847 Erosion ReductionE=$1,309,239*sq mile green roof$442,261

Pollution Removal i-Tree (UFORE) using clipped rasters from Arc GIS ImperviousTree Canopy Land Use Removal CO NO2 O3 SO2 PM10

Benefits of Removed Pollution Total Savings = $21,783 PollutantCOSO2O3NO2PM10 Reduction (tons) Savings$113.1$431.1$12,452.6$3,223.2$5,563.3

Direct Energy Savings Savings CategoryAmount of saving per sq. ft. of green roof area $ saved per sq. ft. of green roof area Direct Energy Savings kWh/sq ft. annually $168,470 CO2 Mitigation tons $22, cents/kWh -Assuming fossil fuel energy production

Benefit Summary Pollutant Reduction$238,847 Erosion Reduction$442,261 Removed Pollution $21, Direct Energy Savings$168,470 CO2 Mitigation$22,264 Total$893,625 *cost of greenroofs $75,338,588

Waller Creek Tunnel Project $127 million dollars expected to reduce the size of the 100-year floodplain of the lower Waller Creek watershed by an estimated 28 acres allow denser development in a very desirable area of downtown a pump station at Waterloo Park will maintain constant water flow in the creek during the dry season to improve water quality

Waller Creek District Master Plan

Conclusions Many benefits to greenroofs both monetary and environmental The City of Austin has an opportunity with new development to implement a large area of greenroofs Other benefits that could be quantified and modeled to make greenroofs a more viable BMP